1. Field of the Invention
The present invention relates to a high-density sintered ITO compact, and an ITO sputtering target for production of transparent electroconductive film, particularly to an ITO sputtering target without nodule formation.
2. Description of the Related Art
The thin film of ITO (indium tin oxide) has been widely used in a varieties of application fields such as display electrodes for a flat panel, soler cell windows, and antistatic films, because it has high electroconductivity and high transparency, and easily to fine photo lithgraph. The demand for the ITO thin film is increasing for pixel electrodes of flat panel displays such as a liquid crystal display which are intended for larger size and finer display quality.
The processes for ITO thin film formation are classified into chemical formation processes such as spray dry pyrolysis, and CVD, and physical formation processes such as electron beam evaporation, and sputtering. Of these processes, sputtering is employed in various application fields because of ease of producing a larger size of films having high performance.
In production of ITO thin film by sputtering, the sputtering target employed includes an alloy target composed of metallic indium and metallic tin (hereinafter referred to as an "IT target"), and a composite oxide target composed of indium oxide and tin oxide (hereinafter referred to as an "ITO target"). Of these targets, the ITO target is widely employed in ITO thin film production because of less variation with time of resistivity and transparency of the resulting film and ease of controlling film formation conditions in comparison with the IT target.
In continuous sputtering of the ITO target in an atmosphere of an argon-oxygen gas mixture, a black matter, called nodules, will appear on the surface of the target with the increase of integrated sputtering time. The black matter, which is considered to be a sub-oxide of indium, is formed on the periphery of the erosion race track of the target, and tends to cause arcing on sputtering and is known to be a source of particles in the formed thin film.
Consequently, during continuous sputtering, defects will be produced by the particles in the formed thin film, which lowers productivity of flat panel displays such as liquid crystal display. In recent years, flat panel displays are intended to be finer and more precise, so that the particles defect occurrence in the film is a serious problem.
In conventional production of the ITO thin film, the nodules formed on the target surface is periodically removed to prevent the occurrence of the defects in the thin film. Since this target cleaning operation lowers remarkably the productivity, an ITO target is required which is less liable to cause the nodule formation.
Many investigations are being made to prevent nodule formation on the ITO target. For example, sintering is conducted in a pressurized oxygen atmosphere of higher than one atmospheric pressure (Japanese Patent Publication 5-30905, etc.). The sintered ITO compact in this method has a remarkably high density of 7.06 g/cm.sup.3. However, for the sintering under high pressure, the furnace should be placed entirely in pressure-tight vessel, which requires large and expensive production equipment, and a larger production apparatus cannot readily be provided, disadvantageously.
The sintered ITO target which has voids of an average diameter of 3-8 .mu.m in a void density of not more than 1500 voids/mm.sup.2 is known to cause extremely less occurrence of arcing, nodules, gas adsorption, etc. (Japanese Patent Laid-Open Publication 5-148636, etc.). This target, however, must be sintered in an oxygen atmosphere of not lower than one atmospheric pressure. Therefore, the same problem is involved on the larger size of the production apparatus.
In another method for retarding the nodule formation, powdery indium oxide and powdery tin oxide having respectively average diameter of not more than 0.1 .mu.m is fired in an oxygen atmosphere at a temperature of 1350.degree. C. or higher, and the resulting fired powdery matter is again crushed, and sintered in the non-oxygen at a temperature of from 500 to 1000.degree. C. under a pressure of not lower than 100 kg/cm.sup.2 to produce a sintered ITO compact causing arcing less (Japanese Patent Laid-Open Publication 4-160047, etc.).
The above method of production of a sintered ITO compact requires firing of a mixture of powdery starting materials, and subsequent crushing, molding and sintering. Therefore, the production process is complicated, and the production efficiency is low. Moreover, the sintered compact density is as low as about 94% in relative density, thus sufficient effect of increasing the sintered compact density being not readily achievable.
An object of the present invention is to provide a high-density ITO sputtering target which does not cause nodule formation on the target surface, thereby preventing defects in an ITO thin film formed by sputtering for uses such as transparent electrodes of flat panel display.
After comprehensive investigation to solve the above problem, it was found by the inventors of the present invention that the amount of the nodules is closely related to the number of arcing generated on the target surface during sputtering.
On the basis of the above findings, the inventors of the present invention investigated further the influence of voids in the interior of the sintered compact on the arcing frequency and the formation of nodules, and found the facts below:
(1) The arcing frequency and the amount of the formed nodules depend on length of the voids size in the sintered ITO compact, and a sintered ITO compact having average length of void size of not larger than 0.7 .mu.m causes less arcing frequency and less formation of nodules. PA0 (2) With the sintered ITO compact having average length of void size of not larger than 0.7 .mu.m, the arcing frequency and the nodule formation are further decreased when the sintered compact has a density of not less than 7.08 g/cm.sup.3, and/or a maximum length of void size of less than 5.0 .mu.m (measured by the diameter method). PA0 (1) Tin oxide segregates around the voids in the interior of the sintered compact. PA0 (2) The size of the voids in the sintered compact depends on the size of segregated tin oxide, and the void diameter in the sintered compact decreasing with decrease of the size of segregates of tin oxide. PA0 (3) Larger segregates of tin oxide are observed in the interior of a sintered ITO compact having a low sintering density, and the number of the larger tin oxide segregates decreases with increase of the sintering density. PA0 (4) Sintered grain of extremely large grain size are observed in a sintered ITO compact of a low sintering density, which is considered to be caused by abnormal grain growth of ITO. PA0 (1) The density of a sintered ITO compact is remarkably increased by use of a mixed indium oxide powder, and tin oxide powder which has a maximum size of not larger than 1.0 .mu.m and a median diameter of not larger than 0.4 .mu.m as the starting materials. PA0 (2) When the tap density of the mixed powder of the above Item (1) is less than 1.8 g/cm.sup.3, the strength of the resulting green compact is lower, and may cause cracking of the green compact during sintering. PA0 (3) The sintered density of the green compact composed of the above powder mixture is increased only by sintering in an oxygen atmosphere. The sintered density is much lower when the sintering is conducted in the air or a non-oxidizing atmosphere. PA0 (4) The ITO sputtering target composed of a high-density sintered compact produced from the above mixed powder and having a sintered density of not less than 7.08 g/cm.sup.3 prevents nodule formation significantly more effectively than a conventional target. PA0 (1) A surface of a sintered ITO compact is mirror-polished by means of a rotary polisher, or the like (for example, polished successively with water resisting polishing paper of #220, #500, #800, and #1000) and alumina slurry. PA0 (2) The polished surface of the sintered compact is subjected to a thermal etching treatment or a chemical etching treatment to reveal grain boundaries on the surface. The thermal etching treatment is conducted by heating the sintered compact at 1400.degree. C. for 5 minutes in an electric furnace. The chemical etching treatment is conducted by immersing the sintered compact in aqua regia for a time ranging from 1 to 10 minutes at room temperature. PA0 (3) The etched surface of the sintered compact is observed by a scanning electron microscope (SEM), and SEM photographs are taken at randomly selected 5 or more portions of the surface at magnification of about 1000 to about 5000. PA0 (4) At least 200 voids are arbitarily picked up from the SEM photographs, and the sizes are measured by a diameter method. Here, the term "each size measured by a diameter method" means the largest length of each respective voids in the arbitarily same direction in the SEM photographs. The average length of the void size is derived by measuring the largest lengths in the same direction of each voids.
Further investigation was made by the inventors of the present invention on the sintered compacts of ITO which have various relative density to invenstigate the mechanism of sintering of ITO, and obtained the findings:
After further investigation, it was found by the inventors of the present invention:
Experiments were further conducted on the basis of the above findings regarding the dependency of the sintered density and the voids in the interior of the sintered compact on the particle size of the tin oxide powder and the particle size of the indium oxide powder. It was found that a sintered ITO compact having a high sintered density can be obtained by use of starting materials of the particle diameters within certain ranges, and the resulting sintered compact is useful as a target for sputtering of an ITO thin film production. Consequently the present invention has been accomplished.